It is well known that dimethylbenzenes, i.e. xylenes has three isomers, namely para-, meta- and ortho. The para-isomer is industrially more important than that of the other two isomers.
1,4-dimethylbenzene, i.e. para-xylene is useful in the manufacture of terephthalic acid that is an intermediate in the manufacture of polyester fibre. The production of para-xylene is a multi million dollar business, and the large scale of the economies involved mean that even a small improvement in these processes results in improving the cost effectiveness of the process. Dimethylbenzenes, i.e. xylenes can be conveniently prepared by employing Friedel Crafts alkylation catalyst like AlCl3, HCl, HF, BF3 etc. However, these catalysts are corrosive in nature. In addition, it is impossible to avoid loss of raw material through multiple alkylations and other side reactions. Further, the xylenes are produced in thermodynamic equilibrium composition, e.g. para-; meta-; ortho-=24:52:24. These three xylene isomers have very close boiling point to each other, the relative volatility is nearly one. Hence separation of para-xylene is difficult and tremendously expensive.
Mobil Oil Corporation discovered a new type of zeolite known as ZSM-5. The method of preparation of this zeolite is described in U.S. Pat. No. 3,702,886. The crystal structure of ZSM-5 zeolite has a specific order of arrangement and is a porous aluminosilicate material. The specific pore size and regular channels have the capability to absorb or allow entry of such molecules as are smaller in size than that of the pore-opening, while rejecting larger molecules. Hence, it is frequently referred as ‘molecular sieve’. In addition, ZSM-5 zeolite also exhibits property of shape-selectivity. The phenomenon of shape selectivity has been described in detail in “Shape-selective Catalysis in Industrial Application”, Vol.36, Mercel Dekker Inc. (1989).
There are many precedents in industry, which make use of these characteristics to conduct chemical reactions. ZSM-5 catalyst is characterized by its selectivity, being able to satisfy the needs for high selectivity to products of different molecules, but its selectivity falls short of expectation in respect of isomers of same kind of product. For instance, when toluene is alkylated with methanol over ZSM-5 catalyst, selectivity for xylenes is very high, but the ratio of isomers of xylenes namely para-, meta- and ortho-, remains near thermodynamic equilibrium composition. The details are reported in J. of Catalysis, Vol.67, page 159 (1981), by W. W. Keading et. al.
Enhancement of para-selectivity, (the fraction of para-isomer in a mixture of para-disubstituted aromatics), by treatment with organosilicon compound is usually referred to in the art as selectivation by silanation. The organosilicon compound is usually known as selectivating agent. The method normally comprises contacting the zeolite with organosilicon compound, separation/removal of solvent (if used), and calcination of zeolite to deposit silica or polymeric silica as a layer on the zeolite.
It is known in the art that the efficiency of silica deposition in order to enhance the selectivity of the zeolite depends on the nature or the kind or the type or the molecular structure of the selectivating agent, i.e. the organosilicon compound employed. The efficiency of silica deposition also depends on the temperature of silanation, the solvents or the carrier for the organosilicon compound, the method or procedure adopted for the selectivation. Pretreatment of the zeolite, i.e. treatment carried out before selectivating the zeolite has also been found to affect the final selectivity of the zeolite. Also post-treatment, i.e. treatment after the selectivating the zeolite have also been described in the art to further improve the selectivity of the zeolite for particular hydrocarbon conversion processes.
Selectivation of zeolites by silanation can be vapour phase or liquid phase. Liquid phase silanation is also referred as ex-situ silanation, or ex-situ selectivation. The zeolite is impregnated with an organosilicon compound dissolved or dispersed in a carrier or solvent followed by calcination of such treated zeolite in an oxygen containing atmosphere under conditions sufficient to remove organic material therefrom and deposit siliceous material on the zeolite. Such ex-situ silanation may result in deposition of at least 1% by weight of siliceous material on the catalyst or zeolite.
Examples of various patents, which teach the ex-situ selectivation of zeolites to enhance para-selectivity are U.S. Pat. No. 3,698,157 (to Allen et. al.), U.S. Pat. No. 4,002,697 (to Chen), U.S. Pat. Nos. 4,127,616 and 4,402,867 (both to Rodewald).
U.S. Pat. No. 3,698,157 (to Allen et al) describes improved chromatographic separation of C8 aromatic mixture for the recovery of para-xylene therefrom using aluminosilicate zeolite H-ZSM-5 modified with octadecyltrichlorosilane.
U.S. Pat. No. 4,002,697 (to Chen) describes preparation of catalyst for xylene manufacture by toluene methylation. Silica modified catalysts employed for the purpose were based on zeolites like ZSM-5, ZSM-11 or ZSM-21 of average crystal size of greater than 0.5μ, having surface deactivated by reaction with compounds of nitrogen or silicon, i.e. phenyl carbazole or dimethyldichlorosilane, (which are sufficiently large as to be unable to penetrate the pores of said crystalline aluminosilicate) followed by calcination. Pyridine was employed as a solvent for dimethyldichlorosilane.
U.S. Pat. No. 4,127,616 (to Rodewald) describes catalysts suitable for alkylation of toluene with methanol or ethanol, and toluene disproportionation to obtain selectively the corresponding dialkyl benzene. The catalyst was prepared by deposition of large organosilicon compound e.g. polymeric phenylmethyl silicone or polymeric methylhydrogen silicone on crystalline aluminosilicate H-ZSM-5, followed by calcination.
Silica modified zeolite catalysts have been described in U.S. Pat. No. 4,402,867 (to Rodewald), utilizing aqueous emulsion of methylhydrogen silicone. Such catalysts contain added amorphous silica within the interior of crystalline structure of the zeolite. The organosilicon compound employed in this patent is small enough to enter the pores of the zeolite.
When the ex-situ selectivation process is repeated more than once, the procedure is referred to as multiple selectivation or multiple silanation. In multiple selectivation method, the zeolite is treated at least twice, generally from two to six times with a liquid medium containing the organosilicon compound(s). In the multiple selectivation method, the zeolite is calcined after each impregnation of the organosilicon compound. Examples of multiple silanation are found in U.S. Pat. No. 4,060,568 (to Rodewald), U.S. Pat. Nos. 4,283,306 and 4,449,989 (both to Herkes), U.S. Pat. No. 5,349,114 (to Lago et.al), U.S. Pat. No. 5,495,059 (to Beck et.al), U.S. Pat. No. 5,552,357 (to Lago et.al), U.S. Pat. No. 5,574,199 (to Beck et.al.), U.S. Pat. Nos. 5,726,114 and 5,990,365 (to Chang et.al).
Modification of zeolites described in U.S. Pat. No. 4,060,568 (to Rodewald), comprises preparing crystalline aluminosilicate zeolite catalyst containing amorphous silica within the interior crystalline structure of ZSM-5, by exposing the zeolite to a volatile silane of small molecular dimension, which preferably enters the pores of zeolites, followed by treatment with aqueous ammonia and calcination. The patent describes a catalyst modified by three such treatments with intermediate calcination after each treatment, but provides no description of any enhancement in catalytic selectivity or activity over that which might follow from a single such treatment.
U.S. Pat. Nos. 4,283,306 and 4,449,989 (both to Herkes) also describe methods of modifying crystalline silica catalyst by application of such silica sources as tetraethylorthosilicate (TEOS), or phenyl methyl silicone. Interestingly, performance of the catalyst treated once with a TEOS solution followed by calcination, was better than that of catalyst treated twice with TEOS, and calcined after each treatment. It showed that twice treated catalyst is less active and less selective than the once treated catalyst as measured by methylation of toluene by methanol, indicating that multiple ex-situ silanation confers no advantage over single silanation, rather results in a adverse effect on the para-dialkyl benzene selectivity.
U.S. Pat. No. 5,349,113 (to Chang et al) describes modification of molecular sieve catalyst by treating with substantially aqueous solution of a water soluble organosilicon compound. The method includes concurrent preselectivation and activation to get activated catalyst. The invention also comprises in-situ selectivation by passing a high efficiency para-xylene selectivating agent along with the reactants.
U.S. Pat. No. 5,349,114 (to Lago et al) describes shape-selective hydrocarbon conversion over modified catalytic molecular sieve, which has been modified by (i) being preselectivated with a first silicon containing compound and (ii) subsequently steamed at about 280° C. to 400° C. The patent indicates that the molecular sieve is modified in as-synthesized conditions.
U.S. Pat. No. 5,495,059 (to Beck et al) also describes multiple ex-situ selectivation sequence employing an aqueous carrier for the organosilane compound. Each sequence includes an impregnation of the molecular sieve with the selectivating agent and a subsequent calcination of the impregnated molecular sieve.
Selectivation of molecular sieves has been described during extrusion by agglomerating with organosilicon compound by Chang et al in U.S. Pat. No. 5,541,146.
U.S. Pat. No. 5,552,357 (to Lago et al) describes catalyst modification by treatment of ZSM-5 in as-synthesised or in ion-exchanged form, first by treatment with a silicon containing polymer (propylamine silane polymer) in substantially aqueous solution, followed by calcination. The catalyst was further in-situ selectivated with a second silicon containing compound. For multiple ex-situ selectivation during first stage, i.e. during treatment with propylamine silane polymer, the catalyst was calcined after first treatment and before the second treatment.
Post-treatment of selectivated zeolite with a dealuminizing agent, e.g. monovalent or polyvalent acids, triethylene diamine, urea, ethylenediamine tetra acetic acid, ammonium hexafluorosilicate has been described in U.S. Pat. No. 5,567,666 (to Beck et al).
U.S. Pat. No. 5,574,199 (to Beck et al) describes shape-selective aromatization with a catalytic molecular sieve, which has been modified by multiple ex-situ selectivation method. The method involves exposing the catalytic molecular sieve to at least two selection sequences, each sequence comprising contacting the catalyst with dimethylphenylmethyl polysiloxane in a solvent, followed by calcination.
U.S. Pat. No. 5,726,114 (to Chang et al.) describes a method for modifying intermediate pore catalytic molecular sieve by multiple ex-situ selectivation process by contacting the zeolite with an aqueous emulsion comprising of a silicon containing selectivating agent stabilized with the aid of surfactant and calcining the contacted molecular sieve after each impregnation of silica. The method further comprises of mild steaming of the silica deposited zeolite and also in-situ trim selectivation of the ex-situ selectivated zeolite.
U.S. Pat. No. 5,990,365 describes a method for preparation of a catalyst comprising ZSM-5, rhenium and a selectivating agent e.g. either coke or siliceous material or a combination thereof. The multiple selectivation is carried out by (i) combining a bound form of zeolite with an organosilicon compound (ii) calcining the organosilicon containing material to remove organic material therefrom to deposit siliceous material on the bound ZSM-5 and (iii) repeating steps (i) and (ii) at least once.
While the above mentioned art is of interest, there is no suggestion of enhancing the selectivity of metallosilicate by treatment with aqueous water after the zeolite has been contacted with organosilicon compound and before calcination of the zeolite to improve the silanation efficiency. There is also no suggestion in any of the prior art known to the applicants of multiple silanation of metallosilicates without any intermediate calcination of organosilicon compound treated zeolite after each silanation. Additionally, there is no suggestion of recycling the solvents/carriers for multiple silanation.
Therefore, it would be a significant advance and improvement in the art to overcome the difficulties, disadvantages and deficiencies associated with conventional methods and procedures for modifying catalytic metallosilicates, molecular sieves modified by such methods and the process of shape selective hydrocarbon conversion using such modified catalytic molecular sieves.
The present invention seeks to solves the difficulties, disadvantages, and deficiencies faced by the prior art by providing an improved method for modifying catalytic metallosilicate molecular sieves, and improved processes for shape selective hydrocarbon conversions.